Semantic Definition Format (SDF) modeling for Digital Twin

4.1. Introduction of sdfContext

A new SDF keyword sdfContext described in [I-D.draft-ietf-asdf-sdf-nonaffordance] is introduced to represent non-functional or metadata elements that describe a device or component without implying direct interaction:¶

• Identifier (e.g., UUID, URN)¶

• Location (e.g. site, zone, GPS tag)¶

• Owner (e.g., representative, ,anufacturer)¶

These field can appear in both sdfObject and sdfThing contexts and follow the same structural pattern as sdfData and is designed for scalability.¶

4.2. Digital Twin Modeling using elements of an SDF model

To support hierarchical representations (e.g., a boat composed of heater, GPS, and battery subsystems), this document encourages use of sdfThing to aggregate related sdfObject components, along with metadata.¶

The example mapping of digital twin attributes to SDF elements is shown in Table 1.¶

4.3. Relationship modeling

The sdfRelation, defined in [I-D.draft-laari-asdf-relations], is a structure for specifying logical or physical relationships between objects within an SDF model. If conventional sdfThing, sdfObject, and sdfProperty focus on defining the properties of individual digital twins, sdfRelation is a means of expressing interactions and structural links between them. Since these relationships go beyond a single digital twin definition, they must be managed in a separate structure, where sdfRelation is used. The sdfRelation keyword allows describing complex relationships beyond just the parent-child hierarchy. These relationships can include:¶

• Physical relations (e.g., "inside", "next to")¶

• Functional relations (e.g., "controls", "is controlled by")¶

• Semantic relations (e.g., "similar to", "same as")¶

The sdfRelation definition can include the following fields as defined in [I-D.draft-laari-asdf-relations]:¶

• relType: Specifies the type of relationship that can an external ontologies (e.g., SAREF[saref4bldg]) can refer to.¶

• target: Points to the SDF object or an external ontology term that is the target of the relationship.¶

• description: Provides a detailed textual explanation of the relationship.¶

• label: A short human-readable label for the relationship.¶

• property: Additional properties describing the relationship context.¶

• $comment: Optional properties including implementers notes.¶

An example of sdfRelation is shown in Figure 2. The sdfProtocolMap in this example is defined in [I-D.draft-ietf-asdf-nipc] and [I-D.draft-ietf-asdf-protocol-mapping]¶

{
   "sdfThing": {
      "Room001": {
          "description": "Contains lightbult and thermostat"
           "sdfObject": {
               "lightbulb": {
                 "description": "A smart lightbulb",
                 "sdfProperty": {
                     "adjacent-node": { "type": "object", "sdfType": "link"}
                  },
                  "sdfRelation": {
                     "sameRoomAsThermostat": {
                        "relType": "saref:isLocatedIn",
                        "target": "#/sdfObject/thermostat",
                        "description": "This lightbulb is located in the same room as the thermostat.",
                        "label": "Located together"
                     }
                  }
             },
             "thermostat": {
               "description": "A thermostat is in the same room as the lightbulb",
               "sdfProperty": {
                  "adjacent-node": {"type": "object","sdfType": "link"}
                }
              },
              "sdfProtocolMap": {
                "description": "Protocol between the lightbulb and thermostat",
                "ble": {
                  "services":
                      [{"serviceID": "361c9c4f-22d7-4a1e-824b-8b61045a566a"}],
                  "cached": false,
                  "cacheIdlePurge": 3600,
                  "unit": "Second",
                  "autoUpdate": true,
                  "bonding": "default"
              }
            }
         }
     }
  }
}

5.1. Overview

Domain-specific examples are provided to illustrates how SDF-based Digital Twin representations can be modeled across various application areas. Each example is structured using the sdfThing construct to represent the physical entity, with associated sdfObject, sdfProperty, sdfAction, sdfEvent, and optional sdfContext or sdfRelation entries. These examples cover multiple domains such as maritime, smart building, healthcare, and energy systems, enabling standardized modeling and interoperability across diverse use cases.¶

5.2. Marine system

Table 2 illustrates how various components of a maritime vessel—specifically Boat007—can be represented as a structured Digital Twin using the Semantic Definition Format (SDF) model. Each physical component, such as a heater or battery, is abstracted as an sdfObject, while the overall vessel is modeled as an sdfThing.¶

For each component, key SDF elements such as sdfProperty, sdfAction, or sdfEvent are defined to describe the operational and contextual aspects of the system. Relationships such as sdfRelation are used to express functional connections (e.g., the battery is connectedTo the controller), enabling richer modeling of interactions between components.¶

This structure allows developers and systems integrators to:¶

• Seamlessly capture and communicate device capabilities and states.¶

• Integrate operational data for real-time monitoring and analysis.¶

• Enable interoperability with other domains through standardized semantics.¶

In the context of Boat007, shown in Figure 3, such a Digital Twin can support various applications, including predictive maintenance, energy optimization, and fleet-level coordination, demonstrating the practicality and scalability of SDF-based Digital Twin modeling for mobility and transportation systems.¶

{
  "sdfThing": {
      "boat007": {
      "label": "Boat #007 with a heater",
      "description": "Contains heaters, fans, battery, etc."
      "sdfProperty": {
         "status": {
            "type": "boolean",
            "description": "Indicates if the boat is powered"
          }
      },
      "sdfObject": {
         "heater1": {
          "description": "A heater ",
          "identityManifest": {
             "manufacturer": "HeaterTech Inc.",
              "model": "HEATER-2025-V1",
              "firmwareVersion": "1.4.3",
              "dateOfManufacture": "2025-04-20T09:00:00Z",
              "certifications": [
                { "scheme": "KS", "certId": "KS123", "region": "KR" } ]
            },
            "contextSnapshot": {
              "thingId": "heater:unit5689",
              "timestamp": "2025-05-23T10:20:00Z",
              "installationInfo": {
                 "room": "kitchen",
                  "floor": 1,
                  "mountType": "freestanding",
                  "installationDate": "2025-06-01"
              },
              "usageProfile": {
                  "type": "residential",
                  "powerCircuit": "230V@60Hz",
                  "energyRating": "A++"
              },
              "location": {"lat": 35.1796, "lon": 129.0756 }
            },
            "sdfProperty": {
                "status": {
                  "type": "boolean"
                  "description":"Whether the heater is powered"
                },
                "temperature": {
                   "type": "number",
                   "unit": "degreeCelsius",
                   "description": "Temperature of the heater"
                  }
            },
            "sdfAction": {
                "turnOn": { "description": "Activate the heater" },
                "turnOff": { "description": "Deactivate the heater" }
            },
            "contextPatch": {
                "thingId": "heater:unit5689",
                "timestamp": "2025-06-20T09:00:00Z",
                "location": {"lat": "35.2988", "lon": "129.2547" },
                "installationInfo": {"floor": 1, "mountType": "wall" }
            }
          },
          "thermostat": {
               "maintenanceSchedule": {
                  "timestamp": "2025-05-20T10:00:00Z"
                  "description": "Last maintained date"
              }
          },
          "batterySensor1": {
            "sdfProperty": {
                "chargeLevel": {
                  "type": "number",
                  "unit": "percent",
                  "description": "Battery charge level"
                },
                "voltage": {
                  "type": "number",
                  "unit": "volt",
                  "description": "Battery voltage"
              }
            }
          },
          "powerController1": {
             "sdfAction": {
                "connect": {"description": "Connect power from the battery" },
                "disconnect": {"description": "Disconn power from the battery"}
                }
            }
        },
        "sdfRelation": {
          "temperature-control": {
             "source": "#/sdfObject/heater1/sdfProperty/temperature",
             "target": "#/sdfObject/thermostat1/sdfProperty/setPoint",
             "relationType": "regulatedBy",
             "directionality": "unidirectional",
             "description": "The current temperature of the heater is regulated by the thermostat's setPoint value."
          },
          "battery-to-controller": {
             "source": "#/sdfObject/batterySensor",
             "target": "#/sdfObject/powerController",
             "relationType": "connectedTo",
             "directionality": "unidirectional"
         }
      }
    }
  }
}

5.3. Healthcare system

This example represents a digital twin for a patient health monitor system (patientMonitor001) assigned to a patient. The system reports real-time health properties while referencing contextual patient information with the components and elements shown in Table 3.¶

A digital twin example of a patient monitoring system with ECG and infusion pump components is illustated in Figure 4. in the healthcare domain, where a biosensor measuring the heart rate is functionally connected to an alert system that emits a high heart rate warning. This enables real-time patient monitoring in medical environments.¶

{
  "sdfThing": {
    "patientMonitor001": {
    "sdfObject": {
        "ecg": {
            "sdfProperty": {
                "heartRate": { "type": "number", "unit": "bpm" },
                "rhythmType": { "type": "string" }
              }
          },
          "infusionPump": {
              "sdfProperty": {
                "flowRate": { "type": "number", "unit": "ml/h" },
                "volumeRemaining": { "type": "number", "unit": "ml" }
              }
          }
        },
        "sdfContext": {
            "wardLocation": { "const": "ICU-5A" },
            "patientID": { "const": "PT123456" }
        },
        "identityManifest": {
          "manufacturer": "MediTech",
          "model": "IM-500",
          "serialNumber": "MT-IM500-00789"
        },
        "sdfRelation": {
          "heartRate-to-alertSystem": {
              "description": "The heart rate data from the biosensor is monitored by the alert system, which triggers a warning event when a high heart rate is detected.",
              "source": "#/sdfObject/biosensor/sdfProperty/heartRate",
              "target": "#/sdfObject/alertSystem/sdfEvent/highHeartRateAlert",
              "relationType": "monitoredBy",
              "directionality": "unidirectional"
          }
      }
    }
  }
}

7.1. Overview

It is essential to define a standardized implementation procedure to ensure interoperability, scalability, and effective lifecycle management across digital twin systems. This section outlines a step-by-step approach aligned with the Semantic Definition Format (SDF) model and its architecture, enabling consistent modeling, integration, and operation of digital twins in IoT environments. A general principles for representing an sdfThing as a digital twin within a specific domain is outlined as follows:¶

• defining a purpose for expressing the observable object as a digital twin in the domain¶

• collecting and mapping data based on the roles of the observable object in the domain¶

• configuring the observable object into the digital twin based on the data for the purposes¶

• interworking among digital twins reflecting various roles of the observable object¶

• synchronizing the observable object and the digital twin¶

7.2. Procedure

The procedure of digitally twinning the space and the objects contained in it is described.¶

• Identifying and scoping physical assets: The first step is to clearly identify the physical assets that will be represented as digital twins. This step includes assigning a globally unique identifier, such as a URN or UUID, and determining the extent of modeling. It also involves deciding whether the unique identifier will cover the entire system or focus on a specific subsystem or component. Although all assets in space can be represented by digital twins, it is cost-effective to select assets for implementation purposes and configure them as digital twins.¶

• Defining a digital twin: A detailed digital twin should be defined using SDF structures, including sdfThing and sdfObject. This step requires specifying affordances such as sdfProperty, sdfAction, and sdfEvent, as well as non-affordance metadata like location, owner, and other descriptive elements through sdfContext.¶

• Metadata and contextualization: This step adds metadata that enriches the context of the digital twin, such as geographic location, ownership details, manufacturing information, and feature summaries. It can also support advanced analytics and management, including contextual attributes such as production schedules or maintenance periods.¶

• Binding interfaces and communications: Digital twins are bound to real-world communication interfaces and protocols such as MQTT, CoAP, and HTTP. This allows affordance of SDF models to interact with real-world data sources, APIs, and physical assets in a smooth and reliable manner.¶

• Verification and compliance: Once an asset is defined and bound as a digital twin, it should be validated against syntax and semantic rules using tools such as JSON schema validators or CDDL definitions. Compliance with specific SDF profiles or domain-specific standards must also be verified to ensure interoperability.¶

• Deployment and registration: After verification, the digital twins are deployed in a digital twin registry, edge system, or cloud infrastructure. This step involves registering the model with the discovery service for integration and use by other systems or stakeholders.¶

• Runtime monitoring and updating: During operations, digital twins need to continuously monitor real data and update their status accordingly. Properties updates, event processing, and partial updates using contextPatch messages should be supported for efficient and lightweight synchronization.¶

• Lifecycle and governance management: The life cycle of the digital twin is managed through version tracking, audit logs, and compliance documents. This step ensures safe and transparent governance and enables proper disposal and deregistration when assets are no longer available.¶

10.1. Normative References

[I-D.draft-ietf-asdf-nipc]

Brinckman, B., Mohan, R., and B. Sanford, "An Application Layer Interface for Non-IP device control (NIPC)", Work in Progress, Internet-Draft, I-D.draft-ietf-asdf-nipc-13, 20 September 2025, <https://datatracker.ietf.org/doc/html/I-D.draft-ietf-asdf-nipc-13>.

[I-D.draft-ietf-asdf-protocol-mapping]

Mohan, R., Brinckman, B., and L. Corneo, "Protocol Mapping for SDF", Work in Progress, Internet-Draft, I-D.draft-ietf-asdf-protocol-mapping-01, 16 October 2025, <https://datatracker.ietf.org/doc/html/I-D.draft-ietf-asdf-protocol-mapping-01>.

[I-D.draft-ietf-asdf-sdf-nonaffordance]

Hong, J. and H. Lee, "Semantic Definition Format (SDF) Extension for Non-Affordance Information", Work in Progress, Internet-Draft, I-D.draft-ietf-asdf-sdf-nonaffordance-01, 21 September 2025, <https://datatracker.ietf.org/doc/html/I-D.draft-ietf-asdf-sdf-nonaffordance-01>.

[I-D.draft-laari-asdf-relations]

Laari, P., "Extended relation information for Semantic Definition Format (SDF)", Work in Progress, Internet-Draft, I-D.draft-laari-asdf-relations-04, 28 January 2025, <https://datatracker.ietf.org/doc/html/I-D.draft-laari-asdf-relations-04>.

[I-D.ietf-asdf-sdf]

Koster, M., Bormann, C., and A. Keränen, "Semantic Definition Format (SDF) for Data and Interactions of Things", Work in Progress, Internet-Draft, draft-ietf-asdf-sdf-25, 13 October 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-asdf-sdf-25>.

[ISO23247-1]

"Automation systems and integration Digital twin framework for manufacturing - Part 1: Overview and general principles, ISO 23247-1.", October 2021, <https://www.iso.org/standard/75066.html>.

[ISO23247-3]

"Automation systems and integration Digital twin framework for manufacturing - Part 3: Digital representation of manufacturing elements, ISO 23247-3.", October 2021, <https://www.iso.org/standard/78744.html>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[Y.4600]

Union, I. T., ""Recommendation ITU-T Y.4600 (2022), Requirements and capabilities of a digital twin system for smart cities.", August 2022.

10.2. Informative References

[saref4bldg]

Poveda-Villaln, M. and R. Garcia-Castro, "SAREF extension for building", 5 June 2020, <https://saref.etsi.org/saref4bldg>.